Cellulose is a principal constituent of all plant matter and is an abundant, naturally renewable material. The form in which the cellulose appears varies widely, including cotton, which is nearly pure cellulose, and wood, which contains approximately 50 to 70% cellulose on an oven-dry basis. Ruminants, such as cattle and sheep, are capable of ingesting cellulosic material in large quantities, however, the amount of useable food value varies widely. The cellulose of woody plants, straw, rice hulls, bagasse, etc., in contrast to that of grasses and legumes, is largely unaffected by the ruminant digestive processes. This is due to the cellular lignin bonding which acts as a cellulose masking agent. As a consequence, woody plants, straw, rice hulls, corn cobs, bagasse, etc. have little available digestible nutrients to cattle, horses, sheep, goats, and swine.
In an effort to utilize the cellulosic materials, certain processes have been developed for increasing the digestibility of wood cellulose by chemically breaking the lignin cellulose bonds. An example is the process disclosed in U.S. patent application, Ser. No. 654,555, filed Feb. 2, 1976, and now U.S. Pat. No. 4,053,645. In the process of this application, plant matter having a low level of digestibility is mixed with water, nitric acid, and a non-oxidizing acid. For suitable reaction, the plant matter must first be finely divided. Sawdust, as from a lumber mill, is in ideal condition for processing without further physical treatment; however, straw tree bark, limbs, and the like should be divided or chipped into particles. The present invention is directed to manufacturing methods employing the basic concepts of this pending application, and particularly is directed to a process of increasing the digestibility of cellulosic material employing a continuous, as opposed to a batch, process.